[sync/server] wtc race maybe cause zero worker thread

[yuqitao]

Description of problem

wtc race maybe cause zero worker thread here.

ttrpc-rust/src/sync/server.rs

Line 111 in 152ac12

if c > max {

could reproduce it by add sleep 1s before

ttrpc-rust/src/sync/server.rs

Line 112 in 152ac12

wtc.fetch_sub(1, Ordering::SeqCst);

Expected result

There is at least one worker thread for the accepted socket fd.

Actual result

Mabye there is no worker thread for the accepted socket fd.